I really don't get the 4 party size, it's really sad and fun breaking.
The main issue is that you NEED a tank + a heal + you if you aren't you of the two mandatory (no I won't make mindfucking metabuilds).
Thus you are left with only one spot forcing you to miss a lot of fun and even whole skills.
I usually play a paladin two handed crusader type, I need a tank, often a warrior + a healer, often a cleric => then either I take a magic user or a ranged physical dmg dealer, but can't both and not even a fun last character like a bard or something.

Pathfinders did it, with success on two games, you probably could as well....
Therefore I'm using a mod even if it will unbalance the game because you know we are playing for fun mainly.